1. Field of the Invention
The present invention relates to a thermosensitive recording material, and more particularly to an improved thermosensitive recording material comprising a support and a thermosensitive coloring layer formed thereon, which is colored by the application of heat thereto.
2. Discussion of Background
Generally, a thermosensitive recording material prepared by forming on a support, such as a sheet of paper or a film, a thermosensitive coloring layer which comprises as the main component a thermasensitive coloring composition is capable of producing a colored image by heating with a thermal head, thermal pen, laser beam or stroboscopic lamp. This type of recording material is advantageous over other conventional recording materials because the recording can be speedily achieved by a simple process with a relatively compact device. Noise development and environmental problems can be minimized, and the manufacturing cost of the recording material is low. Owing to the above-mentioned advantages of the thermosensitive recording material, it is utilized in a wide number of fields such as facsimile machines, electronic computer terminals, recorders and automatic vending machines for labels and tickets.
The thermosensitive coloring composition for use in such a thermosensitive recording material comprises a coloring agent and a color developer capable of inducing color formation in the coloring agent upon application of heat thereto. Specifically, a colorless to light-colored leuco dye with a lactone or lactam, or a spiropyran ring is employed as the coloring agent. An organic acid or phenolic material is conventionally used as the color developer. The recording material employing the combination of the above-mentioned coloring agent and color developer is capable of producing images with clear tone, while maintaining a high degree of whiteness of the background. In addition, the weather-resistance of the obtained images is superior.
However, the thermosensitive recording material has the shortcomings that the recorded images easily fade when brought into contact with water or plasticizers such as dioctyl adipate (DOA) ad dioctyl phthalate (DOP), and that the background of the recording material is easily colored when brought into contact with oil, or solvents such as ethanol and ethyl acetate. The reliability of the images recorded on the thermosensitive recording material is not satisfactory.
To solve the aforementioned reliability problem, the provision of a protective layer on the thermosensitive coloring layer has been proposed as disclosed in Japanese Laid-Open Patent Applications 54-128347 and 54-3594. In addition, it is disclosed in Japanese Laid-Open Patent Applications 56-126193 and 56-13993 that the protective layers comprising water-soluble resins, for example, vinyl alcohol or modified materials thereof, and starch or modified materials thereof are effective in improving resistance to oil and plasticizer of the obtained thermosensitive recording material.
However, the water resistance of such a protective layer comprising the water-soluble resin is poor, so that the protective layer is easily peeled off when brought into contact with water. To improve the water resistance, the protective layer is often prepared by curing with a crosslinking agent as is outlined in Japanese Laid-Open Patent Application 57-188392. Various combinations of the water-soluble resin and the crosslinking agent are conventionally proposed.
For instance, the combination of polyvinyl alcohol and an epoxy compound with two or more epoxy groups or an ethylene-imine compound with two or more ethylene-imine groups is proposed in Japanese Laid-Open Patent Application 60-68990. The combination of casein and a compound having a methylol group has been proposed in Japanese Laid-Open Patent Application 60-193693; the combination of a copolymer of acrylamide--acrylonitrile--a carboxyl-group-containing vinyl compound, and an epoxy resin in Japanese Laid-Open Patent Application 61-37467; and the combination of polyvinyl alcohol and a 2,4-dihydroxy-1,4-dioxane derivative in Japanese Laid-Open Patent Application 62-44494. Furthermore, bis(4-glycidyloxyphenyl)sulfone is used as the crosslinking agent as disclosed in Japanese Laid-Open Patent Application 62-270382.
However, the pot life of coating liquids for the protective layer comprising some of the above crosslinking agents is short because these coating liquids tend to thicken with time, and the crosslinking efficiency of such a crosslinking agent as an ethylene-imine compound deteriorates with time. Although the pot life of the coating liquids is not short, the water resistance is insufficient.
In addition, the method of using the water-soluble resin and the crosslinking agent in combination to prepare a coating liquid for the protective layer has the shortcomings that the sensitivity of the obtained recording material is decreased, and the matching properties of the recording material to a thermal head are degraded. Therefore, the thermofusible material of the recording material adheres to the thermal head and consequently, the recorded images become blurred.